A few people have had this issue of WiFi being very unreliable. For me, what got it to work was setting it in 802.11n-only mode for both 2.4GHz and 5Ghz bands.
I suggest the following:
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Upgrade to latest firmware 1.07.16
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Do a complete factory reset to and then enter all settings from scratch
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In WiFi settings, change to 802.11n only mode
By default the WiFi setting is b/g/n for 2.4GHz and a/n for 5GHz. When I left it at this default, I experienced a lot of WiFi disconnections and then refusal to reconnect. When I switched to N-only, WiFi connections became much more stable.
My suspicion is that the router and devices are getting confused somehow. Most devices are also multi-protocol compatible. What might be happening is that if both router are b/g/n compatible, they might be trying to communicate via different protocols and getting mixed up. By setting it to N-only, there is no confusion.
I should note that under old firmware 1.05.12, this problem was much less pronounced for me even with all protocols enabled by default.